Electronic Information Media

Magnetic recording technology

All magnetic tape media consist of three components:

• ferromagnetic recording material, capable of being magnetised when placed in a magnetic field;

• substrate or base material, on which the recording material is coated; and

• a binder which functions as a carrier for the recording material, and bonds it to the substrate.

The magnetic materials used in audio recordings and video recordings are properly described as ferromagnetic. They are characterised by strong, easily detectable, spontaneous and permanent magnetisation—even without an external magnetic field.

Audio information, for example, speech or music, is recorded in the magnetic layer as a continuously varying analogue sound signal. The magnetic signal is made by an electromagnet, which conveys variations in electrical strength from the output of a microphone to the recording medium. On playback, the original sound is reproduced by reversing the process and replacing the microphone with a loudspeaker.

Video information, for example, the moving images filmed at weddings or on holidays, is recorded in the magnetic layer as a continuously varying analogue signal. Sound information on videos is recorded in the same way, but only in narrow tracks along the edges of videos.

The magnetic signal is made by a magnetic field. It can be erased deliberately when a new recording is made. But it can also be altered accidentally by a magnetic field that gets close enough to the recorded signal to alter it.

If magnetised material gets hot enough, the magnetism disappears. The point at which this occurs varies with the recording material. For example, with iron alloys it is 77ºC.

Recording materials

The International Electrotechnical Commission—IEM—classify audio tapes into three types: Types I, II and III. A tape's classification is determined by the recording material used.

The gamma form of ferric oxide, iron rust, is the most widely used recording material. It is the recording material for audio cassettes designated as Type I. They are typically described as normal bias tapes.

Chromium dioxide was introduced in the late 1960s as a magnetic material suitable for high-density recording. These are known as IEC Type II—high bias—audio cassettes.

Pure iron particles, the recording material used by so-called metal media, are used in IEC Type III audio cassettes and digital audio tape—DAT—cassettes. They support recording densities approximately three times stronger than gamma ferric oxide particles.

Substrates

With magnetic tapes, the substrate is typically plastic film.

The base materials of early magnetic tapes, some of which may be stored in libraries and archives, were composed of cellulose triacetate or polyvinyl chloride—PVC.

Substrates of newer magnetic tape invariably consist of polyethylene teraphthalate—PET—which is often identified by one of its trade names, such as Du Pont's Mylar or Eastman Kodak's Estar and is known in the film industry as polyester. Compared with earlier substrate materials, PET films are stronger and more resistant to high temperatures and humidity.

Binders

Early magnetic tapes featured polyvinyl chloride—PVC—binders. Today, polyethylene binders are commonly used. These binders don't stand up well to high humidity which softens the binder. If the binder has softened to the extent that the particles either move or come right off the base, permanent damage will have been done to the recording. In a dry environment, it is possible that the binder may be re-hardened by reverse hydrolysis to get the sound back.